Light emitting devices using a light emitting diode (LED) are mainly composed of a combination of an LED chip as an excitation light source and a fluorescent material. Further, a variety of luminescent colors can be achieved depending on the combination.
For white LED light emitting devices for emitting white light, a combination of an LED chip for emitting light in a blue range with a fluorescent material is used. Examples of the combination include, for example, a combination of an LED chip for emitting blue light with a mixture of fluorescent materials. As the fluorescent material, a yellow fluorescent material is mainly used which emits light complementary to blue, and with the LED chip, used as a pseudo white light LED. Besides, three-wavelength white LEDs have been developed which use an LED chip for emitting blue light, a green to yellow fluorescent material, and a red fluorescent material.
It is desirable for white LED light emitting devices to be used for lighting purposes (hereinafter, referred to as white light emitting devices) to achieve high color rendering properties, in particular, a high average color rendering index (Ra) in order to reproduce a color close to natural light. In addition, for lower power consumption, the devices require a balance with a high luminescent efficiency.